110 kg of aluminum and steel: in the heart of the 1,800 cc BMW Boxer

The bore measures per stroke are 107.1 and 100 mm with light alloy pistons with the combustion chamber partially obtained in the sky of these pistons, while the crankshaft is obtained from tempered and forged steel; given the dimensions and the considerable weights at stake for all the various components of the frame, the crankshaft has three supports instead of the usual two that characterize the boxer engines. The cylinder head has four valves per cylinder with a diameter of 41.2 mm for the intake and 35 for the discharge and are inclined by 21 ° at the inlet and 24 ° at the outlet. The distribution is with rods and rocker arms to have heads with a smaller lateral development to avoid touching the curves with the two cylinders with a capacity of almost a liter soon. Articulated distributions such as the variable timing of the last 1250 boxers require more space while this is really reduced to a minimum, there is no automatic system to restore the operating clearance which is entrusted, as in the past, to a simple screw and a locknut, just like old boxers. The camshafts are at the top of the engine and take their motion from the crankshaft via a chain and two gears with a pattern that repeats that of the R 5 engine. Lubrication has an important part in this engine because the oil it also serves to eliminate heat: 4.1 liters is the quantity of lubricant, practically the same volume (4 liters) as the last 1250 boxers and there is a double delivery and recovery pump and a large radiator in front of the engine. The fuel supply is by injection with a 48 mm diameter throttle body and an injector placed practically in the intake duct. Together with the jets of air taken from the filter case, a “vortex” effect is combined in the cylinder head which improves combustion, therefore lowers pollution (we are in a Euro 5 regime) and decreases consumption (declared in 5-6 liters of fuel per 100 km). The transmission department is also different from the other schemes that we find in the new boxers because the clutch is dry, with a single disc combined with a large flywheel and combined with a control system when climbing: a sort of anti-hop device that minimizes the reactions of the final shaft transmission. The six-speed gearbox also has four axles to lower the transmission output and has (optional) reverse gear. The result of this remarkable work is a boxer engine that delivers the power of 91 HP (67 kW) at 4,750 rpm and a remarkable maximum torque of 158 Nm at 3,000 rpm, but at 2,000 rpm we already have 150 Nm. there are countershafts to reduce vibrations, the boxer scheme for its own construction minimizes the problem, indeed the German engineers have insisted on saying that the engine is “alive” and that its pulsations are healthy and beautiful “good vibration”.


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